U.S. Pat. No. 3,918,889 issued Nov. 11, 1975, discloses a so-called in-line flame arrestor added as a module mounted on the burner housing which was, in turn, mounted on the end of a firetube. The thinking at that time of conception was that the spools of flame arrestor elements could be made as large in diameter as required to pass enough combustion air to supply the size of burner selected. It was contemplated that the burner protrude back through a vertical wall to pick up primary combustion air, secondary combustion air being supplied through wall openings controlled by louvers. Although there were many advantages to this arrangement, it proved somewhat awkward to link the louvers to the fluid supply. Also, the two separate modules of flame arrestor and burner housing proved somewhat awkward as an extension of structure to hang on the entrance of a firetube.
Now that the in-line type flame arrestor has been generally accepted, the awareness of the present inventors has been enlarged to encompass consolidation of the two modules, plus a gain in simplicity of access to the burner and combustion air controls for inspection, repair and replacement.